You can ask the builder to add a new field to an existing table without deleting and recreating the table.
Describe the field, its type and what existing records should contain. For example: Add an optional estimated_value number to the items table. Keep all existing records and fields.
An optional field can start empty. A new required field needs a default value so existing rows have a value. Adding a column is different from changing an existing column's type or changing the table's access rules; ask the builder to explain those changes separately.
Export important data before changing the schema. Afterwards, inspect existing records and try saving a new record. Test any screens that now use the new field.
Keep uploaded files in file storage and store their URL or key in the table. Do not put an image, video or its base64 content into a text column.
Builder messages use credits. See Making a data table private for the separate access-control question.